A mutant beta-tubulin confers resistance to the action of benzimidazole-carbamate microtubule inhibitors both in vivo and in vitro

The mutant BEN210 of Physarum polycephalum is highly resistant to a number of benzimidazole carbamate agents, including methylbenzimidazole-2-yl-carbamate and parbendazole. The resistance is conferred by the benD210 mutation in a structural gene for beta-tubulin. This mutant allele encodes a beta-tubulin with novel electrophoretic mobility. We have used this strain to determine whether the mutant beta-tubulin is used in microtubules and whether this usage permits microtubule polymerisation in the presence of drugs both in vivo and in vitro. In vitro assembly studies of tubulin purified from the mutant strain have shown that microtubules are formed both in the absence of drugs and in all drug concentrations tested (up to 50 microM parbendazole). In contrast, the assembly of microtubules from wild-type tubulin in vitro is totally inhibited by 2-5 microM parbendazole. Thus the resistance of BEN210 to parbendazole observed in vivo has been reproduced in vitro using tubulin purified from the mutant strain. Electrophoretic analysis of the microtubules formed in vitro has shown that both the wild-type and the mutant beta-tubulin are incorporated into the microtubules and that the proportion of mutant to wild-type beta-tubulin appears to remain constant with increasing drug concentration. This is the first demonstration of a single mutation in a tubulin structural gene causing an altered function of the gene product in vitro.